We show that Arabidopsis FHY3 and FAR1, which encode two proteins related to Mutator-like transposases, act together to modulate phyA signaling by directly activating the transcription of FHY1 and FHL, whose products are essential for light-induced phyA nuclear accumulation and subsequent light responses. FHY3 and FAR1 have separable DNA binding and transcriptional activation domains that are highly conserved in Mutator-like transposases. Further, expression of FHY3 and FAR1 is negatively regulated by phyA signaling. We propose that FHY3 and FAR1 represent transcription factors that have been co-opted from an ancient Mutator-like transposase(s) to modulate phyA-signaling homeostasis in higher plants.

We next used a yeast one-hybrid assay to delineate the DNA sequences to which FHY3 and FAR1 bind. GAD-FHY3 or GAD-FAR1 fusion proteins (GAD, GAL4 transcriptional activation domain), but not GAD alone, activated the LacZ reporter genes driven by the FHY1 and FHL promoters. Deletion analysis narrowed down the FHY3/FAR1 binding site to a 39-bp promoter subfragment located on the "a" fragment for both FHY1 and FHL. Notably, these subfragments share a stretch of consensus sequence, 5'-TTCACGCGCC-3'. Mutating the core sequence "CACGCGC" of this motif (m2 and m3 for FHY1, m5 for FHL) abolished the reporter gene activation by both GAD-FHY3 and GAD-FAR1. Mutating the flanking sequences (m1 and m4) did not obviously affect the reporter gene activation by GAD-FAR1, but clearly reduced activation by GAD-FHY3. Thus, "CACGCGC" likely defines a cis-element that confers specific binding for FHY3 and FAR1 and is named FBS for FHY3-FAR1 binding site.